ABSTRACT
In Chapter 3, we saw that growth under very low supersaturation would not occur by surface nucleation followed by spreading of two-dimensional layers. Frank and his collaborators showed that under such conditions, dislocations would be the controlling factor in crystal growth. Frank [1] pointed out that the emergence of a screw dislocation on a crystal face produces on that face a step of height equal to that of the corresponding Burgers vector. A principal crystal face that contains the end of screw dislocation will appear essentially like the one shown in Figure 5.1. It will have a step whose termination is not at the boundary but at the emergence point of the dislocation. The growth of the crystal on such a surface does not eliminate the step because if we assume that the crystal is growing by the attachment of molecules to the edge of this step, the step in question would be self-perpetuating and continues to be present on the crystal surface as long as the dislocation line intersects the surface. Since the step provided by the screw dislocation terminates at the dislocation line, it can advance only by rotating around the dislocation. Hence, the step winds itself into a closed spiral centered on the dislocation, and as the growth proceeds the spiral revolves around, as shown sequentially in Figure 5.1. This will also be true for any dislocation whose Burgers vector has a component normal to the face in which the dislocation terminates. Unlike the ideal crystals of “n” layers, the crystal containing a screw dislocation consists of only one layer in the form of a helicoid. When the dislocation is of multiple strengths, we have a crystal of several interleaved helicoidal layers.
